1. Field of the Invention
This invention relates to an improved method of petroleum coking. In one aspect, this invention relates to a method of producing consistent high quality coke. In another aspect, this invention relates to a method of producing coke from feedstocks which have not been used previously as coker feedstocks.
2. Prior Art
Delayed coking is a well-known process used to convert heavy hydrocarbonaceous feedstocks to petroleum coke. Coking is used to process refinery streams, such as heavy residua, which cannot economically be further distilled, catalytically cracked or otherwise processed to make fuel-grade blend streams or streams which can be processed to make fuel-grade blend products. Heavy hydrocarbonaceous coker feedstocks are typically atmospheric residuum, vacuum residuum, catalytic cracker residual oils, hydrocracker residual oils and other residual oils from other refinery units.
Crude oil comprises hydrocarbons and hetero atoms, including heterocyclic compounds having non-metallic elements such as oxygen, nitrogen, sulfur, phosphorous, selenium and others, in complex ring structures, and compounds containing metals such as iron, vanadium and nickel. Sulfur-containing and metal-containing compounds, which are naturally occurring crude, can result in sulfur and/or metals contamination of coke product, adversely impacting coke uniformity and quality. These sulfur and metal-containing contaminant compounds become concentrated in residua which is used as coker feedstock, since the contaminant-containing compounds generally have relatively high boiling points and relatively complex molecular structures. Process economics dictate against separation or removal of the contaminant-containing compounds from the residua which is fed to a coker.
Coker feedstock is generally not just one residuum; it is a mixture of residua, which mixture varies depending upon refinery operations. Because of changes of composition and rates of flow of the various streams throughout an operating refinery, concentrations of sulfur-containing compounds and metal-containing compounds in the residua and coker feedstock change.
In delayed coking, a heavy hydrocarbonaceous feed for the coker is heated in a preheater external to the delayed coking drum to a temperature in the range of about 900.degree. F. to about 1000.degree. F., and the heated stream is fed to the coking drum. During the coking cycle, the coke drum is maintained at delayed coking conditions at a pressure in the range of about 20 psia to about 60 psia , and a temperature in the range of about 900.degree. F. to about 1000.degree. F. for a period of about 15 to about 30 hours, and the heavy hydrocarbonaceous feed is thermally cracked in the drum to form porous, solid coke and to form lighter hydrocarbons, which are vaporized and removed overhead from the drum during coking and are passed to a coker fractionator and recovered. Although some compounds containing sulfur and metals are removed during coking, the coke so formed in prior art processes generally contains residual sulfur and metals in amounts varying corresponding to variations of the amount of sulfur and metals found in the residua fed to the coker. At the end of the coking cycle, the feedstream is switched from the first drum to a second parallel coke drum, while the coke in the first drum is stripped by steam or other stripping media to remove recoverable hydrocarbons entrained or otherwise remaining in the coke, and the drum is cooled by steam or other cooling media to reduce the temperature of the drum while avoiding thermal shock to the coke drum and then is quenched by water or other quenching media to rapidly lower the drum temperature to conditions favorable for safe coke removal. The bottom and top heads of the drum are removed from the drum, and the coke is cut, typically by hydraulic water jet, and is removed from the drum. After coke removal, the drum heads are replaced and the drum is preheated and otherwise readied for the next coking cycle.
With such prior art delayed coking processes, coke quality is inconsistent and has a variable content of contaminants, including sulfur- and metal-containing compounds. Without consideration of coke quality, the coker is fed variable residua and other oils that are available in the refinery to be fed to the coker. The coke so formed varies with variations in the feed, including variations in the relative mixes of the residua of which the feed is comprised, relative compositions of the residua in the mixes, and in particular, variations in the concentrations of contaminants, such as sulfur and metals, in the residua.
There is a need for a method to control coke quality and to make high quality and consistent quality coke. While low and variable quality coke can be burned as fuel, high quality and consistent quality coke is desirable for certain industrial applications, such as anode-grade coke used in making consumable graphite electrodes useful in aluminum production.